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simple question for CMOS switch

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ella1923

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hi i would like to ask how to determine transistor sizes in the figure below for a switching speed of 20MHz.

thank you very much
 

for better response, all the transistor should be big enough so that the on-resistance is as small as possible. however, since your freq operation is 20Mhz, then the transistor must not too big because if the parasitic capacitance is very big, then the output will be spoiled. in my opinion, for this case, try-and-error is the best method to go.
 

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    ella1923

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Azmi said:
for better response, all the transistor should be big enough so that the on-resistance is as small as possible. however, since your freq operation is 20Mhz, then the transistor must not too big because if the parasitic capacitance is very big, then the output will be spoiled. in my opinion, for this case, try-and-error is the best method to go.
Click to expand...

Correct. Also, the source and load impedance need to be taken into account, as does the digital drive and inverter drive characteristics. Large transistors could be switched at 20MHz but would require a high gate drive current at speed.

Keith.
 

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    ella1923

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i will be using the switch as shown in the figure below. i have tried trial and error method but i only reach at 2MHz and the output is not that good. do you think it has something to do also with my opamp slew rate? is it possible to have 20MHz freq operation without redesigning the OP?

38_1264737787.jpg




thank you very much.

Added after 2 minutes:

oh i forgot, M6 is a power MOS.

thanks in advance.
 

I suggest you try to isolate which part is the problem. Try the opamp on its own with your 20MHz signal and no load. Is it fast enough? Then add the MOSFET. Assuming all is ok, then test the switch on its own, slowly adding bits until the performance becomes unsatisfactory.

It looks like the opamp is probably a problem even before you add the switch. If you need a switch to simulate it, you could always add a "perfect" one before then trying the analogue switch.

Keith.
 

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    ella1923

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keith1200rs said:
I suggest you try to isolate which part is the problem... Try the opamp on its own with your 20MHz signal and no load. Is it fast enough? ...Keith.
Click to expand...
You see, a Pwr MOS hase input C of 1-2 nF or more...
I dont belief that your OP can drive it, if; than not up to 20MHz, THIS IS your PROBLEM!
You can select dedicated MOS driver ICs, or built a good output driver-or is it a ASIC?
Check for some Applications for "High Speed Driver circuits...
LT, ADI, NS APEX_ all has some solutions.
K.
 

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    ella1923

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check your OP whether its slew rate is enough to charge the load and make sure that your output impedance of OP is low-Z. for your CMOS switch, just simply run simulation on switch itself using sin wave at 20Mhz and see what happen on the output before and after the switch turned on.
 

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    ella1923

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thank you very much for all of your replies..yep, i really need to check my OP performance.
 

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